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I Geotechnical Engineering Report Addendum 1 Micropile Foundations—Evergreen Way Pump Station E Everett,Washington lierracon August 2, 2021 E Terracon Project No. 81195189 a factor of safety of 2 for allowable stress design. Alternatively, we recommend a value of 0.55 for the resistance factor in the strength limit state and 1.0 in the extreme (seismic event) limit state, in both axial compression and tension, for LRFD design. Drilled and grouted piles having a diameter less than about 12 inches are generally considered to be micropiles. Terracon recommends a minimum pile nominal diameter of 6 inches. Terracon's experience is that installers in the area typically have equipment capable of installing 6- and 8-inch diameter piles; larger diameters may be available as well. The bond stress values above represent the expected load capacity of the adhesion between the grout and the adjacent soil. Due to the small end area, the tip resistance of micropiles is typically neglected when the micropile tip bears on soil and should be neglected for this design. We also recommend that the micropiles be spaced at a minimum spacing of 30 inches on center. MICROPILE INSTALLATION RECOMMENDATIONS We recommend that the micropiles be installed by a competent specialty foundation contractor ' experienced with this type of construction. All micropile shafts should be drilled using drilling equipment capable of advancing through dense to very dense advance outwash soils. Groundwater was not encountered within the depth of the borings at the time of drilling; however, micropiles are anticipated to extend deeper than the explorations. Groundwater levels fluctuate with the season, precipitation, site use, and other factors. Casing may be required to support the borehole. Cobbles and boulders are often encountered in advance outwash. Contractors should be prepared to drill or core through cobbles and boulders. Bentonite slurries, if used to stabilize the borehole could adversely affect grout to soil bond capacity. We recommend the plans and specifications prohibit the use of bentonite slurries. After the hole is drilled to the planned depth, all cuttings must be removed from the hole. Means and methods of drilling and flushing the borehole should be selected by the contractor to cause the minimal practicable disturbance or upheaval to the ground, existing structure, floor slabs, and pavements. The contractor should prevent loss of ground during micropile installation. Micropile installation should take place immediately upon completion of drilling. Grout should be placed from the bottom of the borehole up by the tremie method followed by pressure grouting. Reinforcing steel should be installed, with centralizers. Post-grouting may be necessary to achieve the recommended soil- to-grout bond values. TESTING AND OBSERVATION We recommend pre- andpost-condition surveys u eys of the existing pump station building. Pre-condition surveys should be performed prior to beginning any drilling activities on the site. Post-condition survey Responsive a Resourceful Q Reliable 2 I